Select the range of supposed email addresses and use F&R, Regular expression enabled with Current selection only.
Enter exactly ^[^@]+$ in the field behind “Find:”, the order Find All.
(RegEx can also “search negative”. If you need to understand, study Regular Expression Tutorial - Learn How to Use Regular Expressions, The + in place of the * used by @JohnSUN avoids to find empty cells.)
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For what you need to do, filtering might be the better means:
Select the column containing the email addresses, go >Data>More Filters>Standard Filter The dialog that opens will offer the variant Does not contain under Condition, and you only need to enter the @ then under Value.
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